The present invention relates generally to electrical fuse circuit designs, and more particularly to designing a tamper-resistant electrical fuse circuit for security applications. Electrical fuse is a convenient logic nonvolatile memory for permanently holding information such as “chip-ID”, etc. A typical implementation is a laser-fuse, where laser energy is used to program the fuse by evaporating metal or polysilicon links and the resulting resistance change is sensed using a latch. However pitches of the laser-fuse device are not scalable below the wavelength of the laser beam, typically 1.06 um, thus the laser-fuse is not suitable for deep submicron technologies.
To overcome the laser-fuse's pitch limitation, an electrical fuse, typically made of silicided polysilicon, uses electrical current instead to program. When programming the electrical fuse, a high current density, typically 600 mA/um2 for the silicided polysilicon, is applied to the electrical fuse link for a certain period of time. The resistance of the electrical fuse will rise due to the electrical stress in its fuse link. A few micro seconds of stress may be a long enough time to cause a discernable resistance change, ideally more than 1 Kohm, in the electrical fuse.
FIG. 1 is a schematic diagram illustrating a conventional electrical fuse circuit 100, which comprises a programming block 110 and a sensing block 120. The programming block 110 is implemented as a PMOS transistor 113 coupled between an electrical fuse element 122 and a programming power supply VDDQ. When a gate signal SL of the PMOS transistor 113 is turned low, this particular electrical fuse element 122 is selected for being programmed, i.e., a programming current will flow through it. After being subject to the programming current for a certain period of time, the resistance of the electrical fuse element 122 will rise. The sensing block 120 which comprises a reference resistor 124, PMOS transistors 132 and 136 and NMOS transistors 134 and 138, is to sense the resistance level of the electrical fuse element 122. The sensing block 120 then outputs a logic state at a node Q through an inverter 142. A logic LOW at node Q may correspond to a programmed electrical fuse element 122, i.e., electromigration stressed. On the other hand, a logic HIGH at node Q corresponds to a not-programmed electrical fuse element 122.
However this conventional electrical fuse circuit 100 has limitations. First, the current that is used to stress the conventional electrical fuse 122 is normally quite high and may cause visible changes, such as cracks, in the fuse link. Therefore, data stored in the electrical fuse circuit 100 can be detected through visual inspections which render it unfit for security applications, such as smart cards and micro code storage.
Second, when the PMOS transistor 113 is turned on and a relatively small VDDQ voltage, for instance 300 mV, is applied, the electrical fuse element 122 will not be stressed enough to cause its resistance to rise. But in the conventional fuse circuit 100, since the VDDQ voltage supplies only to the electrical fuse element 122, the resistance value (R) of the electrical fuse element 122 can be read out by measuring a current (I) that flows through the electrical fuse element 122, and calculating R=VDDQ/I, presumably the gate of MOS 132 is turned off. Therefore, data stored in the conventional electrical fuse circuit 100 can also be read out electrically through an externally accessible programming channel without altering the data. For this reason the conventional electrical fuse circuit 100 cannot be used for security applications either.
Third, the conventional sensing block 120 may not be sensitive enough in low voltages to distinguish the small resistance change in the electrical fuse element 122 caused by the electromigration. Referring to FIG. 1, the NMOS transistor 134 is biased into saturation region by the NMOS transistor 138. This bias voltage is determined by a voltage divider formed by the PMOS transistor 136, the NMOS transistor 138 and the reference resistor 124. A resistance variation of the electrical fuse element 122 changes the source voltage of the NMOS transistor 134 which causes a current flowing through the PMOS transistor 132 to vary. In this configuration, the PMOS transistor 132 operates in low impedance state of linear region. If so, the gain of the sensing block 120 is low, and the sensing block may not be sensitive enough to detect small resistance change in the electrical fuse element 122, thereby failing its function.
As such, what is desired is an electrical fuse circuit that can be programmed electrically, yet data stored therein cannot be optically observed or electrically read out by unauthorized means.